The economics of petroleum production and refining are requiring that more usable materials be obtained from petroleum residues of ever worsening characteristics, primarily sulfur content, metal content and asphaltene content of the petroleum residues, resulting from the atmospheric and vacuum distillation of petroleum feedstocks. The distillation of the petroleum feedstock tends to concentrate the contaminants into the petroleum residue.
Common ways of improving the yield of distillate products and disposing of the residue have involved hydrotreating. Hydrotreating involves reacting the petroleum residue with hydrogen in the presence of a catalyst to convert the petroleum residue into a higher proportion of more valuable lower-boiling products. The residue remaining after the lower-boiling products are removed from the hydrotreater effluent generally has a lower sulfur and metal content.
Another process commonly used to treat petroleum residue is delayed coking. In this process, the petroleum residue is heated and subjected to destructive thermal cracking to produce valuable lower-boiling petroleum distillate products, and forming a solid carbonaceous residue known as coke. Coke with a high sulfur and/or metal content is generally subject to combustion as a fuel. "Fuel grade coke" is not generally suitable for other purposes.
Higher quality coke grades such as anode grade coke generally have lower sulfur and metal content. For example, anode grade coke generally has a sulfur content less than 3 weight percent, a nickel content less than 200 ppm, a vanadium content less than 350 ppm and a total metals content less than 500 ppm. In addition, anode grade coke which is suitable for use as making a carbon anode which can be used in aluminum manufacture, for example, must also have an HGI grindablility index greater than 70, a bulk density of at least 50 lbs/ft.sup.3, and a volatile carbonaceous material content of less than 10 or 12 weight percent. It is more desirable to produce anode grade coke since this is a higher value product than fuel grade coke.
Particularly with high sulfur, high metals residues, one approach has been suggested to hydrotreat the residue which removes the sulfur and metal so that the coke obtained by destructive thermal cracking of is the hydrotreated residue is within specifications for anode grade coke. Unfortunately, however, it is known that hydrotreating of the petroleum residue feedstock affects the physical characteristics of the coke, which can make the coke unsuitable for the anode manufacturing process. Therefore, for the production of anode grade coke, feedstocks have been historically limited to virgin residues with inherently low sulfur and metals content. Petroleum residues are generally comprised of saturate, aromatic, resin and asphaltene fractions. Hydrotreating a petroleum residue is known to convert a portion of the resin fraction to saturates. The data below in Table 1 are based on the feed and product from a commercial hydrotreating unit and illustrate this change:
TABLE 1 Resin fraction (wt %) Saturates fraction (wt %) Virgin petroleum 35 14 residue Hydrotreated residue 13 33
It is generally accepted that the type of change in composition illustrated above can make the hydrotreated residue unsuitable for anode grade coke production.
It is also known to subject petroleum residue fractions to solvent extraction to separate the residue fraction into a deasphalted oil fraction and an asphaltene fraction, and sometimes into a third resin fraction. It has been known to hydrotreat and/or catalytically crack the deasphalted oil and/or resin fractions, and treat the asphaltene fraction in a delayed coker. However, as far as applicants are aware, no one has previously tried to improve the quality of coke produced in the delayed coker by feeding the resin-containing fraction from the solvent deasphalting of the petroleum residue to a delayed coker unit.
U.S. Pat. No. 5,013,427 to Mosby et al. discloses hydrotreating a petroleum residue feed with a resin fraction from a solvent extraction unit together in a residue hydrotreating unit, feeding a first portion of the residue hydrotreating unit bottoms fraction to the solvent extraction unit, and a second portion of the hydrotreated residue to a coker unit. Similar disclosures are found in U.S. Pat. Nos. 4,940,529 to Beaton et al.; 5,124,027 to Beaton et al.; 5,228,978 to Taylor et al.; 5,242,578 to Taylor et al.; 5,258,117 to Kolstad et al.; and 5,312,543 to Taylor et al.